Re: [PATCH 2/3] tcp: Implement hash table with indices rather than pointers

[Stefano Brivio <sbrivio@redhat.com>]

On Mon,  4 Dec 2023 14:16:10 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> We implement our hash table with pointers to the entry for each bucket (or
> NULL).  However, the entries are always allocated within the flow table,
> meaning that a flow index will suffice, halving the size of the hash table.
> 
> For TCP, just a flow index would be enough, but future uses will want to
> expand the hash table to cover indexing either side of a flow, so use a
> flow_sidx_t as the type for each hash bucket.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  flow.h | 11 +++++++++++
>  tcp.c  | 34 +++++++++++++++++++++++-----------
>  2 files changed, 34 insertions(+), 11 deletions(-)
> 
> diff --git a/flow.h b/flow.h
> index c2a5190..959b461 100644
> --- a/flow.h
> +++ b/flow.h
> @@ -53,6 +53,17 @@ static_assert(sizeof(flow_sidx_t) <= sizeof(uint32_t),
>  
>  #define FLOW_SIDX_NONE ((flow_sidx_t){ .flow = FLOW_MAX })

In hindsight, while reviewing the functions below: FLOW_MAX should
probably be MAX_FROM_BITS(FLOW_INDEX_BITS) - 1 instead (and those >=
comparisons would happily become >), so that we don't need to have a
"maximum" value that's also not allowed (then, strictly speaking, it's
more than the maximum).

> +/**
> + * flow_sidx_eq() - Test if two sidx values are equal
> + * @a, @b:	sidx values
> + *
> + * Return: true iff @a and @b refer to the same side of the same flow
> + */
> +static inline bool flow_sidx_eq(flow_sidx_t a, flow_sidx_t b)
> +{
> +	return (a.flow == b.flow) && (a.side == b.side);
> +}
> +
>  union flow;
>  
>  void flow_table_compact(struct ctx *c, union flow *hole);
> diff --git a/tcp.c b/tcp.c
> index 09acf7f..7e438b7 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -574,7 +574,7 @@ static unsigned int tcp6_l2_flags_buf_used;
>  #define CONN(idx)		(&(FLOW(idx)->tcp))
>  
>  /* Table for lookup from remote address, local port, remote port */
> -static struct tcp_tap_conn *tc_hash[TCP_HASH_TABLE_SIZE];
> +static flow_sidx_t tc_hash[TCP_HASH_TABLE_SIZE];
>  
>  static_assert(ARRAY_SIZE(tc_hash) >= FLOW_MAX,
>  	"Safe linear probing requires hash table larger than connection table");
> @@ -1197,10 +1197,13 @@ static unsigned int tcp_conn_hash(const struct ctx *c,
>  static inline unsigned tcp_hash_probe(const struct ctx *c,
>  				      const struct tcp_tap_conn *conn)
>  {
> +	flow_sidx_t sidx = FLOW_SIDX(conn, TAPSIDE);
>  	unsigned b;
>  
>  	/* Linear probing */
> -	for (b = tcp_conn_hash(c, conn); tc_hash[b] && tc_hash[b] != conn;
> +	for (b = tcp_conn_hash(c, conn);
> +	     !flow_sidx_eq(tc_hash[b], FLOW_SIDX_NONE) &&

Do we actually need to check for FLOW_SIDX_NONE explicitly? That is,
sidx we get as input here should never be FLOW_SIDX_NONE.

I wonder if it makes sense to take care of the possible "overflow"
outcome from step L4. of algorithm L you mentioned in 1/3. It
*shouldn't* because you're enforcing the minimum size of the hash
table, I wonder if it's a good idea anyway.

> +		     !flow_sidx_eq(tc_hash[b], sidx);
>  	     b = (b + 1) % TCP_HASH_TABLE_SIZE)
>  		;

I respect the fact that this is fundamentally a for loop. :) On the
other hand:

	unsigned b = tcp_conn_hash(c, conn);

	while (!flow_sidx_eq(tc_hash[b], sidx))
		b = (b + 1) % TCP_HASH_TABLE_SIZE);

...would be a bit more readable?

>  
> @@ -1216,7 +1219,7 @@ static void tcp_hash_insert(const struct ctx *c, struct tcp_tap_conn *conn)
>  {
>  	unsigned b = tcp_hash_probe(c, conn);
>  
> -	tc_hash[b] = conn;
> +	tc_hash[b] = FLOW_SIDX(conn, TAPSIDE);
>  	flow_dbg(conn, "hash table insert: sock %i, bucket: %u", conn->sock, b);
>  }
>  
> @@ -1229,16 +1232,18 @@ static void tcp_hash_remove(const struct ctx *c,
>  			    const struct tcp_tap_conn *conn)
>  {
>  	unsigned b = tcp_hash_probe(c, conn), s;
> +	union flow *flow = flow_at_sidx(tc_hash[b]);
>  
> -	if (!tc_hash[b])
> +	if (!flow)
>  		return; /* Redundant remove */
>  
>  	flow_dbg(conn, "hash table remove: sock %i, bucket: %u", conn->sock, b);
>  
>  	/* Scan the remainder of the cluster */
> -	for (s = (b + 1) % TCP_HASH_TABLE_SIZE; tc_hash[s];
> +	for (s = (b + 1) % TCP_HASH_TABLE_SIZE;
> +	     (flow = flow_at_sidx(tc_hash[s]));
>  	     s = (s + 1) % TCP_HASH_TABLE_SIZE) {
> -		unsigned h = tcp_conn_hash(c, tc_hash[s]);
> +		unsigned h = tcp_conn_hash(c, &flow->tcp);
>  
>  		if (in_mod_range(h, b, s, TCP_HASH_TABLE_SIZE)) {
>  			/* tc_hash[s] can live in tc_hash[b]'s slot */
> @@ -1248,7 +1253,7 @@ static void tcp_hash_remove(const struct ctx *c,
>  		}
>  	}
>  
> -	tc_hash[b] = NULL;
> +	tc_hash[b] = FLOW_SIDX_NONE;
>  }
>  
>  /**
> @@ -1263,10 +1268,10 @@ void tcp_tap_conn_update(const struct ctx *c, struct tcp_tap_conn *old,
>  {
>  	unsigned b = tcp_hash_probe(c, old);
>  
> -	if (!tc_hash[b])
> +	if (!flow_at_sidx(tc_hash[b]))
>  		return; /* Not in hash table, nothing to update */
>  
> -	tc_hash[b] = new;
> +	tc_hash[b] = FLOW_SIDX(new, TAPSIDE);
>  
>  	debug("TCP: hash table update: old index %u, new index %u, sock %i, "
>  	      "bucket: %u", FLOW_IDX(old), FLOW_IDX(new), new->sock, b);
> @@ -1289,16 +1294,18 @@ static struct tcp_tap_conn *tcp_hash_lookup(const struct ctx *c,
>  					    in_port_t eport, in_port_t fport)
>  {
>  	union inany_addr aany;
> +	union flow *flow;
>  	unsigned b;
>  
>  	inany_from_af(&aany, af, faddr);
>  
>  	for (b = tcp_hash(c, &aany, eport, fport);
> -	     tc_hash[b] && !tcp_hash_match(tc_hash[b], &aany, eport, fport);
> +	     (flow = flow_at_sidx(tc_hash[b]))
> +		     && !tcp_hash_match(&flow->tcp, &aany, eport, fport);

Same as above about readability (somehow clashing with correctness).

>  	     b = (b + 1) % TCP_HASH_TABLE_SIZE)
>  		;
>  
> -	return tc_hash[b];
> +	return &flow->tcp;
>  }
>  
>  /**
> @@ -3090,6 +3097,11 @@ static void tcp_sock_refill_init(const struct ctx *c)
>   */
>  int tcp_init(struct ctx *c)
>  {
> +	unsigned b;
> +
> +	for (b = 0; b < TCP_HASH_TABLE_SIZE; b++)
> +		tc_hash[b] = FLOW_SIDX_NONE;
> +
>  	if (c->ifi4)
>  		tcp_sock4_iov_init(c);
>  

-- 
Stefano